One of the major problems in electrophotographic copying apparatus, and in particular copying apparatus in which dry thermoplastic toner is used, is the fixing of the toner images on the paper. A large number of processes and devices for fixing toner images by means of heat have already been proposed. These fixing devices may be divided into three main types. The first process is known as radiation fixing. In this process, the toner image is heated by the absorption of irradiated energy, and is thereby fixed on the paper. This process requires considerable energy and has the additional disadvantage that, when a radiation process alone is used, the adhesion between the toner particles and the paper fibers cannot be improved. The second process consists of fixing by hot air. The difficulties arising in this process are mainly those of soiling of the apparatus, and the problem of bringing hot air into contact with the toner image in a definite manner. Finally, the third process is known as contact fixing. In this process, a heated surface is brought into contact with the toner image.
From the point of view of consumption of energy and technical expenditure, contact fixing is the most favorable process, but this process also entails serious problems. Firstly, in contact fixing there is the problem of toner particles being deposited in the form of an image on the fixing surface and being transferred onto the next copy. This so-called offset-effect, which gives rise to "ghost images", is extremely troublesome. To alleviate this problem it has been proposed to provide a fixing roller surface with polytetrafluoroethylene covered with a thin film of silicone oil. While this contact fixing device has proven to be the most successful so far, certain problems do occur. Firstly there are difficulties in achieving even temperature distribution over the length of the roller. The distribution is frequently nonuniform in practice, so that the fixing quality and the wipe strength of the copies obtained are unsatisfactory in many places. In addition a considerable expenditure of energy is still necessary for heating the roller if there is provided on the inside of the roller a heating element the temperature of which lies considerably above the temperature on the outer surface of the roller necessary for fixing.
It has also already been proposed to provide on the inside of the roller, instead of a conventional heating device, an electrical resistance heating device under the polytetrafluoroethylene layer, and if necessary to use this in conjunction with a radiation heat source. These improvements, however, still require a considerable technical expenditure.
There is accordingly a need for a contact fixing device which combines as uniform as possible a temperature over the entire width of the contact fixing surface with as small as possible an energy consumption, and in which the heating element is operated at as low a temperature as possible.